Neurotrophic factors or neurotrophins are a family of small, basic proteins which play a crucial role in the development and maintenance of the nervous system. The first identified and probably best understood member of this family is nerve growth factor (NGF), which has prominent effects on developing sensory and sympathetic neurons of the peripheral nervous system (Levi-Montalcini, R. and Angeletti, P. U., Physiol. Rev. 48, 534-569 [1968]; Thoenen, H. et al., Rev. Physiol. Biochem. Pharmacol. 109, 145-178 [1987]). Although NGF and a number of animal homologs had been known for a long time, including a homolog from the mouse submandibular gland, the mature, active form of which is often referred to as .beta.- or 2.5 S NGF, it was not until recently that sequentially related but distinct polypeptides with similar functions were identified.
The first in line was a factor called brain-derived neurotrophic factor (BDNF), now also referred to as neurotrophin-2 (NT-2) which was cloned and sequenced by Leibrock, J. et al. (Nature 341, 149-152 [1989]). This factor was originally purified from pig brain (Barde, Y. A. et al., EMBO J. 1, 549-553 [1982]), but it was not until its cDNA was cloned and sequenced that its homology with NGF became apparent. The overall amino acid sequence identity between NGF and BNDF (NT-2) is about 50%. In view of this finding, Leibrock et al. speculated that there was no reason to think that BDNF and NGF should be the only members of a family of neurotrophic factors having in common structural and functional characteristics.
Indeed, further neurotrophic factors closely related to .beta.-NGF and BDNF have since been discovered. Several groups identified a neurotrophic factor originally called neuronal factor (NF), and now referred to as neurotrophin-3 (NT-3) (Ernfors et al., Proc. Natl. Acad. Sci. USA 87, 5454-5458 (1990); Hohn et al., Nature 344, 339 [1990]; Maisonpierre et al., Science 247, 1446 [1990]; Rosenthal et al., Neuron 4, 767 [1990]; Jones and Reichardt, Proc. Natl. Acad. Sci. USA 87, 8060-8064 (1990); Kaisho et al., FEBS Lett. 266, 187 [1990]; copending U.S. application Ser. No. 07/494,024 filed Mar. 15, 1990). NT-3 shares about 50% of its amino acids with both .beta.-NGF and BDNF (NT-2). Neurotrophins-4 and -5 (NT-4 and NT-5), have been recently added to the family (copending U.S. application Ser. No. 07/587,707 filed Sep. 25, 1990; Hallbook, F. et al., Neuron 6, 845-858 [1991]; Berkmeier, L. R. et al., Neuron 7, 857-866 [1991]; Ip et al., Proc. Natl. Acad. Sci USA 89, 3060-3064 [1992]). The mammalian molecule initially described by Berkmeier et al. supra, which was subsequently seen to be the homolog of Xenopus NT-4, is usually referred to as NT-4/5.
Neurotrophins, similarly to other polypeptide growth factors, affect their target cells through interactions with cell surface receptors. According to our current knowledge, two kinds of transmembrane glycoproteins serve as receptors for neurotrophins. Equilibrium binding studies have shown that neurotrophin-responsive neurons possess a common low molecular weight (65-80 kDa), low affinity receptor (LNGFR), also termed as p75.sup.NTR or p75, which binds NGF, BDNF, and NT-3 with a K.sub.D of 2.times.10.sup.-9 M, and large molecular weight (130-150 kDa), high affinity (K.sub.D in the 10.sup.-11 M) receptors, which are members of the trk family of the receptor tyrosine kinases.
The first member of the trk receptor family, trkA, was initially identified as the result of an oncogenic transformation caused by the translocation of tropomyosin sequences onto its catalytic domain. Later work identified trkA as a signal transducing receptor for NGF. Subsequently, two other related receptors, mouse and rat trkB (Klein et al., EMBO J. 8, 3701-3709 [1989]; Middlemas et al., Mol. Cell. Biol. 11, 143-153 [1991]; EP 455,460 published Nov. 6, 1991) and porcine, mouse and rat trkC (Lamballe et al., Cell 66, 967-979 [1991]; EP 522,530 published Jan. 13, 1993), were identified as members of the trk receptor family. The structures of the trk receptors are quite similar, but alternate splicing increases the complexity of the family by giving rise to two known forms of trkA, three known forms of trkB (two without functional tyrosine kinase domains) and at least four forms of trkC (several without functional tyrosine kinase domain, and two with small inserts in the tyrosine kinase domain). This is summarized in FIG. 1.
The role of the p75 and trk receptors is controversial. It is generally accepted that trk receptor tyrosine kinases play an important role in conferring binding specificity to a particular neurotrophin, however, cell lines expressing trkA bind not only NGF but also NT-3 and NT-4/5 (but not BDNF), trkB expressing cells bind BDNF, NT-3, NT-4, and NT-4/5 (but not NGF), in contrast to trkC-expressing cells which have been reported to bind NT-3 alone (but not the other neurotrophins). Furthermore, it has been shown in model systems that the various forms of trk receptors, arising from alternate splicing events, can activate different intracellular signalling pathways, and therefore presumably mediate different physiological functions in vivo. It is unclear whether cells expressing a given trk receptor in the absence of p75 bind neurotrophins with low or high affinity (Meakin and Shooter, Trends Neurosci. 15, 323-331 [1992]).
Published results of studies using various cell lines are confusing and suggest that p75 is either essential or dispensable for neurotrophin responsiveness. Cell lines that express p75 alone bind NGF, BDNF, NT-3, and NT-4 with similar low affinity at equilibrium, but the binding rate constants are remarkably different. As a result, although p75-binding is a common property of all neurotrophins, it has been suggested the p75 receptor may also play a role in ligand discrimination (Rodriguez-Tebar et al., EMBO J. 11, 917-922 [1992]). It is unclear whether the p75 receptor alone is capable of mediating neurotrophin biological activity. While the trk receptors have been traditionally thought of as the biologically significant neurotrophic factor receptors, it has recently been demonstrated that in melanoma cells devoid of trkA expression, NGF can still elicit profound changes in biological behavior presumably through p75 (Herrmann et al., Mol. Biol. Cell 4, 1205-1216 [1993]). Recently, Davies et al. (Neuron 11, 565-574 [1993]) reported the results of studies investigating the role of p75 in mediating the survival response of embryonic neurons to neurotrophins in a model of transgenic mice carrying a null mutation in the p75 gene. They found that p75 enhances the sensitivity of NGF-dependent cutaneous sensory neurons to NGF.
Neurotrophins exhibit actions on distinct, but overlapping, sets of peripheral and central neurons. These effects range from playing a crucial role in ensuring the survival of developing neurons (NGF in sensory and sympathetic neurons) to relatively subtle effects on the morphology of neurons (NT-3 on purkinje cells). These activities have led to interest in using neurotrophins as treatments of certain neurodegenerative diseases. Neurotrophins have also been implicated in the mediation of inflammatory pain, and are overexpressed in certain types of malignancies. Accordingly, inhibitors of neurotrophin biological activity have therapeutic potentials, such as in pain medication and as chemotherapeutics in cancer treatment.
In order to better understand the role of trk and neurotrophin action in various human pathological states, it would be useful to identify and isolate human trkB and trkC proteins, and specifically, to determine which forms of trkB and trkC are expressed in the human. Apart from their scientific and therapeutic potentials, such human trk receptor proteins would be useful in the purification of human neurotrophic factors, and in the diagnosis of various human pathological conditions associated with elevated or reduced levels of neurotrophins capable of binding trkB and/or trkC.
It would further be desirable to provide effective inhibitors of neurotrophic factor biological activity. Such inhibitors would be useful in the diagnosis and treatment of pathological conditions associated with neurotrophic factors.